This invention relates to pressure vents for fuel storage tanks, and, in particular, to a pressure valve which is responsive to both high pressure and low pressure situations in a fuel storage tank.
Fuel storage tanks, such as tanks used to store gasoline at gas stations, are subject to varying pressures which can affect the ability of the fuel pumps to operate. The fuel storage tanks are thus provided with vents which allow excess pressure to be released when the tank exceeds a certain predetermined maximum pressure. Excess pressure can be caused by vacuum assist refueling when more vapors are drawn into the tank than fuel is dispensed, by not hooking up the vapor return hose to the transfer tank when refueling the tank, or by atmospheric conditions. In addition, negative pressure or partial vacuums can be created in the storage tank by dispensing more fuel than vapors that are drawn in (during vacuum assist refueling). In such a situation, the tank must be repressurized (i.e., the vacuum must be dissipated).
Typical vent devices use a direct acting poppet valve that is entirely effected by the pressure from the storage tank. Such vent devices use the same poppet valve to respond to over-pressure and under-pressure (vacuum) situations. Thus, there is only one pressure differential setting for the current vent devices. However, it is generally desirable to have a first pressure setting or differential to release excess pressure and a second pressure differential to release or dissipate a vacuum. It is also desirable to have at least two over-pressure settings to release vapors at a first pressure and at a second, higher pressure. However, current vents use the same valve to respond to both low and high pressure situations. Thus, the pressure at which the valves open are the same and often correlate to the lower pressure. This lower pressure leads to a seal which is not as tight as it could be. And, with typical vents, vibrations or minor imperfections on the sealing surface will open or create a leakage path through the poppet valve, and will prevent the vent from maintaining the required pressure in the storage tank.